1. Field of the Invention
The present invention relates to a focus detection apparatus which is used for an image pickup apparatus such as a digital camera or a video camera.
2. Description of the Related Art
A phase difference detection method (hereinafter referred to as “a phase difference AF”) is known as a method of an automatic focus (AF) method in an image pickup apparatus. In the phase difference AF, a light beam passing through an exit pupil of an image pickup lens is divided into two light beams, and the two divided light beams are respectively received by a pair of focus detection sensors. Then, a shift amount of signals outputted in accordance with the light receiving amount, that is, a relative position shift amount in a direction in which the light beam is divided (hereinafter referred to as “an image shift amount”), is detected, whereby a shift amount in the focusing direction of the image pickup lens (hereinafter referred to as “a defocus amount”) is obtained.
Japanese Patent Laid-open No. 2001-305415 discloses a configuration in which an image pickup element is provided with a phase difference detection function, whereby a dedicated focus detection sensor is no longer needed, and a phase difference AF with high speed is realized. In the configuration of Japanese Patent Laid-open No. 2001-305415, a photoelectric converter of a pixel of the image pickup element is divided into two and is provided with a pupil dividing function. The outputs of the two divided photoelectric converters are separately processed to perform the focus detection. Further, the added output of the two divided photoelectric converters is used as an image pickup signal. Japanese Patent Laid-open No. 2007-121896 discloses calculating a conversion coefficient from the distribution of the focus detection optical system and the diameter information of the image pickup optical system, in consideration of a mechanical vignetting by an image pickup lens, whereby the focus detection accuracy may be improved.
However, the difference in the sensitivities of the two divided photoelectric converters is not considered in the configuration of Japanese Patent Laid-open No. 2001-305415. It is difficult to completely equalize the sensitivities of the respective photoelectric converters, and there may be cases where the output of one of the two divided photoelectric converters is saturated, whereas the other is not saturated, especially in an object bright enough for a pixel output (charge) to be saturated.
Accordingly, in order to obtain an accurate light amount, when the output of one of the photoelectric converters is saturated, it is necessary to leak such output to the other photoelectric converter. However, since the image shift amount is calculated by separately processing the output of the two divided photoelectric converters, the accuracy and speed of the focus detection may be influenced. Further, the configuration of Japanese Patent Laid-open No. 2007-121896 calculates the conversion coefficient based on information of the optical system from the image pickup lens to the focus to be detected, so as to improve the focus detection accuracy, although the leakage from the photoelectric converters is not considered.